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Betulaceae


Members of the family Betulaceae are important trees and shrubs in temperate and subarctic latitudes of the Northern Hemisphere, extending to high mountains of the tropics. No species are native in Australia, but several species of birch (Betula) and alder (Alnus) are important temperate garden trees, and occasionally escape from cultivation.

Characteristic features of the family Betulaceae in Australia include:

  • deciduous trees
  • flowers unisexual in separate, distinctive catkins, the males soft and flexuous, the females firmer and often erect
  • perianth segments small, all free, yellow, grey, brown or black; male flowers with 4 stamens; females with several styles on the ovary
  • fruits cone-like or breaking up into papery-winged seeds

Description

Deciduous or semi-deciduous trees. Internal secretions not obvious or of resin. Plants glabrous or with simple, glandular or non-glandular, unicellular or uniseriate hairs. Leaves alternate and spiral, petiolate. Stipules present, distinct and free from the petiole, scale-like or membranous, falling off early. Lamina simple, symmetric, palmatifid or palmatisect or bipinnatifid, tripinnatifid, etc., lanceolate, ovate or orbicular; base cordate; margins crenate, dentate or serrate, ±flat; venation pinnate, with the midrib conspicuous, and the tertiary venation reticulate or not; surfaces not punctate; herbaceous; distinctive odour absent or aromatic. Male and female flowers occurring on the same plant. Inflorescences terminal or axillary, consisting of catkins. Bracts and bracteoles present. Pollination by wind. Flowers sessile. Floral disc absent; nectaries absent. Perianth regular, of 1 whorl only or absent, with 0 or 16, free, ±sepaloid segments, yellow, grey, brown or black, without contrasting markings, papery. Fertile stamens 4, opposite to and free of the perianth segments, free of the ovary and style, distinct from each other, all ±equal. Anthers basifixed, not versatile, opening sideways by longitudinal slits; 2-celled. Ovary superior and sessile. Carpels 2 (3), fused; ovary with 2 (3) locules. Style terminal and branching from the base. Ovules 1 per locule, stalked; placentation apical. Fruit a dry, indehiscent samara or a nut; the perianth on the maturing fruit deciduous or dry and persistent. Disseminule macro-surface winged or costate; micro-surface ±smooth, red or brown, dull. Seeds 1 per fruit. Aril absent. Cotyledons 2. Embryo straight.
(Note: this description has been generated from the coded data compiled for the key. Any errors in the key data will be reflected in the descriptions.)

A treatment of the family Betulaceae has been published in:
Flora of Australia 3: 96.

Australian genera of Betulaceae (as recognised for the Flora of Australia)

* = all species introduced

*Alnus
*Betula


Alnus glutinosa
 


Betula pendula